g2w/grass2world/main.c: Now writes worldfile header to a separate file
named $(world).hdr.  Note that this version of g2w produces world
files that are NOT compatible with versions of RHESSys previous to
version 5.18

g2w/grass2world/sys.c:
  - readheader no longer supports template files with simulation start
    and end date For this reason template files for previous versions
    of RHESSys will not work with RHESSys 5.18's version of g2w.

  - outputheader no longer prints simulation start and end date to
    world file headers

rhessys/include/rhessys.h: Added fields to support new separate world
file header file

rhessys/init/construct_basin.c: Fixed bug where construct_basin
attempted to set null pointer basin->route_list->num_patches to zero

rhessys/init/construct_command_line.c: Added support for '-whdr'
command line field used for explicitly specifying the header to use
for a worldfile. There are three options for where RHESSys will read
world file header from, in order of precedence are:
  1. -whdr command line option
  2. A file named ${WORLDFILE_NAME}.hdr; if -whdr is not specified
  3. From legacy world file (deprecated); if file named
  ${WORLDFILE_NAME}.hdr is not found

rhessys/init/construct_world.c: Implement above 3 options for reading
world file header information.
  - Note that new world file header files do not contain simulation
  start and end dates, but construct_world expects that legacy world
  files (i.e. those that contain headers) will contain simulation
  start and end date.  However, when reading legacy world files, the
  command line value for simulation start and end date always override
  the start and end dates in the legacy world file header.
Merge branch 'develop' of https://github.com/RHESSys/RHESSys into develop